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Build manual · Kratky still water · Leafy greens

Kratky Swiss Chard: two plants, one recharge, three months of cutting

Chard is the best long-cycle crop for a passive bucket. It tolerates a higher nutrient strength than lettuce, regrows from the centre for months, and unlike most leafy greens it does not bolt the moment temperatures wobble. The only change to standard Kratky is one mid-cycle recharge.

By The Grow DeskPublished 8 Oct 2026Difficulty Beginner
System specification90 day cycle
Reservoir
8 Lworking volume
Planting sites
2sites
Pump
nonepassive
Air stones
0not required
EC target
1.6 mS/cmtarget
pH window
5.8 - 6.3pH
Light at canopy
260 PPFDat leaf
Photoperiod
14 hper day
Cycle length
90 daysplant to harvest

Top-up routine — Chard is a cut-and-come-again crop on a long cycle; top up 400 mL of plain pH 6.0 water every 6-8 days and re-charge the bucket once at day 45.

Bill of materials7 line items
Bill of materials
PartSpecificationQtySubstitute
Opaque bucket with lid8 L, 24 cm diameter1A 5 L food container works for one plant; two chard need the extra root volume
Net pots75 mm rim250 mm pots are enough at planting but the stem base thickens by week 8
LECA8-16 mm1.5 LA 50/50 LECA and perlite mix holds slightly more air around the crown
Chard seedFordhook Giant or Bright Lights, 3 seeds per pot6 seedsTransplant a 3-week seedling instead to save ten days
Two-part nutrientLeafy formula, 1.6 mS/cm250 mL eachChard tolerates a general-purpose tomato feed at 1.6 without problems
pH downPhosphoric acid 45 %250 mLChard on hard water will need more acid than lettuce on the same supply
LED bar40 W, 4000 K, 60 cm1A bright south window at 10 cm delivers roughly 260 PPFD for four hours in winter
Assembly, in order6 steps

  1. Sow three seeds per pot and thin hard at day 12

    Chard seed is actually a corky cluster of two or three true seeds, so a single 'seed' often gives three seedlings. Thin to one plant per net pot at day 12 by cutting the extras at the base rather than pulling, which would disturb the root of the one you keep.

  2. Charge at 1.6 mS/cm, higher than lettuce

    Chard is a heavier feeder than lettuce and holds up well at 1.6 mS/cm from the start. On the same 1.2 charge that suits lettuce, chard grows slowly and the leaves stay small and dark. Because the cycle runs 90 days, this higher charge is also what makes the long intervals between recharges work.

  3. Harvest outer leaves from week 6, never the crown

    Cut the outermost 3-4 leaves at the base when they reach 20 cm, leaving at least six leaves on the plant. Chard regrows from the centre, so a plant that keeps its core will produce for three months. Cutting the whole head at once ends the cycle at harvest number one.

  4. Recharge the bucket once, at day 45

    Kratky works on a falling water line, but on a 90-day crop the solution is exhausted well before the plant is. At around day 45, when the level is 4 cm down, top up to the original mark with fresh 1.6 mS/cm solution rather than plain water. This single mid-cycle recharge roughly doubles total leaf yield without changing anything else.

  5. Keep the light at 14 hours year round

    Chard is day-length sensitive enough that winter light levels push it into slow, thin growth. A 14 hour photoperiod at 260 PPFD keeps it producing through December, when a windowsill alone would give it perhaps four usable hours of the intensity it needs.

  6. Watch for leaf miner, not deficiencies

    The main pest on indoor chard is leaf miner, which shows as pale blotches between the leaf veins. Pick and destroy affected leaves immediately rather than trying to treat the plant; on a two-plant bucket, removing four leaves is cheaper than any intervention, and the plant replaces them in ten days.

Week-by-week maintenanceWeek 1-3 · Week 4-6 · Week 7-9 · Week 10-13

Week 1-3

EC 1.6 · pH 6.0

  • Thin to one plant per pot at day 12
  • Keep light 14 h
  • Check roots reach water by day 5

Week 4-6

EC 1.6 · pH 6.1

  • First top-up around day 40
  • Begin harvesting outer leaves at 20 cm

Week 7-9

EC 1.6 recharged · pH 6.0

  • Recharge to full mark with fresh solution at day 45
  • Harvest 2-3 leaves per plant per week

Week 10-13

EC 1.3 falling · pH 6.1

  • Continue cutting outer leaves
  • Remove any leaf with miner blotches
  • Expect a slowdown after day 80

A 90-day crop breaks the standard Kratky rule

The classic Kratky method assumes you charge once and never add nutrient, which works on a 35-day lettuce cycle. Chard runs 90 days on the same bucket, and by day 45 the solution has given up most of its nitrogen and the plant is visibly slowing. One recharge to the original mark with fresh 1.6 mS/cm solution at day 45 keeps the falling water line while restoring the salt balance.

Cut outer leaves and the plant lasts three months

Chard grows new leaves from the centre of the crown continuously. Taking the outermost leaves at 20 cm, three or four at a time and never more than half the plant, keeps the core intact and the harvest going. Cutting the whole plant at once is faster and ends the cycle immediately, since there is no core left to regrow from.

Hard water is the hidden variable

Chard pulls calcium and magnesium heavily and on a hard-water supply the pH in an 8 L bucket will climb from 6.0 toward 6.8 within a week of each top-up, because the plant removes anions faster than cations. Checking pH after every top-up and adjusting back to 6.0 is more important on chard than on any other crop in a small passive bucket.

The leaves get tougher, not bigger, past day 80

From about week 11 the plant is running on a root system that is older than the leaves it is supporting, and the petioles get fibrous while leaf size stays flat. There is no intervention that reverses it. Plan to start replacement seedlings at day 70 so a fresh bucket is ready before the old one stops earning its space.

Tubing diagram: Kratky Swiss Chard: two plants, one recharge, three months of cutting
Tubing diagram: Kratky Swiss Chard: two plants, one recharge, three months of cutting

Key points

  • 1.6 mS/cm start, higher than lettuce, because chard is a heavy feeder
  • Recharge to the full mark with fresh solution at day 45
  • Harvest only outer leaves at 20 cm; never cut the crown
  • Check pH after every top-up; hard water pushes it up fast
  • Start replacement seedlings at day 70 so the next bucket takes over without a gap

Sources

Please noteEC, pH and light figures are general home-system references and will vary with your water supply and nutrient brand. Follow the product label. This article is informational and is not professional horticultural advice.

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